An electronic control unit (“ECU”) mounted on a vehicle is required to control increasingly sophisticated and complicated functions, and it is becoming increasingly difficult to implement all of the required functions on a single ECU. Thus, the need is increasing for executing processes which are originally preferably executed by a single ECU by separate ECUs. However, two processes obtained by dividing processes that are preferably executed by a single ECU need to be synchronized because the processes are closely related to each other.
FIG. 1 illustrates an example of synchronization of two processes. FIG. 1A illustrates a chronological relationship of two processes A and B executed by a single ECU. Because the process A and the process B are closely related to each other, the ECU executes the process A and then starts execution of the process B (after a minimum required time for switching of the processes, for example). For example, the ECU controls an actuator A based on a result of the process A and then controls a related actuator B based on a result of the process B. In this way, the two actuators are controlled in a coordinated manner. Predetermined times (which may be referred to as “control periods”) are allocated to the processes A and B. Namely, a sufficient time for the process A is allocated to a control period A, and a sufficient time for the process B is allocated to a control period B.
FIGS. 1B and 1C illustrate examples of a chronological relationship between the processes A and B where the process A is processed by an ECU_A and the process B is processed by an ECU_B. The ECU_A and the ECU_B are electrically connected to each other. Unless the processes are synchronized between the ECU_A and the ECU_B, a desired chronological relationship between the processes A and B may be broken. For example, a blank time may arise between completion of the process A and the start of the process B (FIG. 1B), or the process B may be started when the process A is not yet completed (FIG. 1C). For example, in the former case, control of an actuator B after the control of an actuator A is delayed. In the latter case, the actuator B may not be properly controlled because the process B is controlled based on the result of the previous process A. Thus, in either case, the actuators A and B cannot be controlled in a smoothly coordinated manner.
Regarding synchronization of processes, a technology has been devised whereby, in a clustered computer system including plural computers, one computer notifies another computer of the start and end of a session in an asynchronous manner (see Patent Document 1, for example). Specifically, a master computer notifies a slave computer of the start and end timings of a parallel process portion.
However, when process synchronization is based on such simple notification, according to the synchronization technology discussed in Patent Document 1, it becomes difficult to synchronize plural computers simply due to the occurrence of communication error at the time of notification.
FIG. 2(a) illustrates an example of a synchronization signal transmitted from the ECU_A to the ECU_B. The ECU_A transmits the synchronization signal to the ECU_B at around the time of completion of the process A. The ECU_B can start execution of the process B by using the synchronization signal as a trigger.
However, communication error may be caused or an interrupt with high priority may be issued during the communication of the synchronization signal, so that the ECU_B may not be able to receive the synchronization signal at all times. If a communication error is caused as illustrated in FIG. 2B, the ECU_B cannot start the process B and has to wait until the ECU_A executes the next process A and transmits the synchronization signal to the ECU_B. Namely, when such a communication error is caused, at least one control period is skipped by the ECU_B. As a result, control of the actuator B is destabilized.
Thus, in the method according to the related art, it has been difficult to synchronize two closely related processes so that the processes can be controlled in a synchronized manner. In this respect, skipping of the process of the ECU_B once may not cause much of a problem in the case of a simple information process not related to control. Further, the problem has not been felt in the related art because there has been a time allowance for the ECU_A to re-transmit the synchronization signal upon detection of communication error.    Patent Document 1: Japanese Laid-open Patent Publication No. 2006-228192